Absorbent articles comprise a plurality of components, like web materials constituting the outer and inner coversheets of the article, absorbent layers etc., which are joined together for example by thermobonding, ultrasonic welding or gluing in a manufacturing process in high speed. Other components like elastic threads, fastening tabs, waist bands, side panels, belts etc. may also be incorporated in articles like diapers, pant diapers and incontinence guards. All these components must be applied and fastened in a correct position of the article. Cutting operations, in which selected lengths of materials are cut off, are also occurring in the manufacture of absorbent articles, and the cut off has to be made in a precise location. Control and synchronization of the joining of the different components in an absorbent article, application of a substrate to a selected area of a component, control of thermobonding, welding and glue patterns as well as of cutting operations, is of critical importance for the product quality.
U.S. Pat. No. 5,045,135 discloses a method for controlling the cut off register in a diaper machine. A diaper web having a plurality of longitudinally spaced apart absorbent pads thereon passes a detection means detecting the edge of the pad. The cut off means is controlled by the signals from the detection means.
EP-A-1 199 057 discloses a method for manufacturing an absorbent article having a predetermined pattern arranged at a predetermined position of the article. The pattern is printed on a continuously moving web material fed at a selected speed and is cut into selected lengths at a predetermined position. The position of the printed pattern is detected and the signal is used for controlling the feeding speed of the web, thereby obtaining the printed pattern at a correct location with respect to the cut off position.
In these two documents an edge of a pad and a printed pattern respectively is visually detected and used for controlling certain process steps. However such product features which are visually detectable at a certain position that may be used for process control are not always available and it therefore exists a need for improvement of the flexibility of the process control.